Various EL thin film elements have hitherto been known, the elements consisting of semiconductors as a luminescent basic material which are compounds of the elements belonged to II-VI groups of the periodic table such as ZnS, ZnSe, CaS, SrS and the like and additives as a luminescence center which are Mn or rare earth elements such as Tb, Sm, Ce, Eu and the like. Applied researches of these EL thin film elements on displays of various kinds of electronic apparatuses, terminals of measuring instruments and computers and planar televisions have been made.
At present, however, only the EL thin films which consist of ZnS as a luminescent basic material and Mn as a luminescence center have been put to practical use. Processes for preparing the El thin films are as follows:
(1) A vacuum evaporation method wherein a sintered pellet of a mixture of ZnS and Mn as a source material is treated by an electron beam heating method (e.g., Japanese Patent Publication No. 10358/1977; and
(2) An epitaxial growth method wherein vaporized Zn and S or Mn are supplied alternately to a substrate whereon a thin film grows in an atomic monolayer unit (e.g., Japanese Patent Publication No. 35158/1982).
Besides the above methods, the following processes for preparing thin films are now under investigation in order to deal with the improvement of film quality or multicolor luminescence:
(3) A radio-frequency sputter method wherein a mixed target of ZnS and Mn is employed;
(4) An organometallic chemical vapor deposition method (OMCVD) wherein an organic compound of II group elements and hydride of VI group elements are combined through thermal decomposition on a substrate; and
(5) A multisource vapor deposition method wherein solid materials of Zn,S and Mn are employed as a vapor source which can be subjected to controllable heat treatment under vacuum independently.
Furthermore, a method of chemical vapor deposition under reduced pressure using gas streams has recently been developed. In this method, solid materials of ZnS and Mn are heated and the heated ZnS vapor and Mn vapor are transported to a substrate placed in low temperature zone by means of a hydrogen gas stream (or an inert gas stream) and a hydrogen chloride gas stream respectively. A growth of the thin film occurs on the substrate through chemical deposition. This method focuses the spotlight of attention as a new process for preparing thin film because it is suitable for mass production of EL thin films having a large area and a high quality (e.g. Japanese Patent Application No. 117943/1988).
In the case where EL panel is prepared by the aforementioned growth techniques of the thin film, it is necessary to produce a homogeneous luminescent layer having a uniform film thickness over the incommensurable large area in comparison with the epitaxial growth of Si or GaAs and the like for the productivity of a semiconductor device. In addition, high productivity of the film is required. Therefore, in the conventional methods such as vacuum evaporation methods, sputter methods and OMCVD methods, which are the methods wherein the substrate on which the thin film is deposited is treated one by one in principle, an apparatus for producing the thin film must be designed in such a way that a mutual arrangement of the glass substrate and the material to be vaporized is optimized in order to deposit the vaporized material on the substrate uniformly. The productivity of these methods is lower than that of the method wherein plural substrates can be treated simultaneously.
Concerning the method wherein plural substrates can be treated simultaneously, there are some reports on an epitaxial growth on a Si wafer having a thickness of a few inches, a growth of amorphous insulating films of Si.sub.3 N.sub.4 and the like and a heteroepitaxial growth of a GaAs system by the CVD method. Furthermore, it is known that a growth of thin films may occur on plural substrates in a box simultaneously (Japanese Patent Application No. 3190/1990). However, there is no newly-developed technique which makes possible a simultaneous growth of ZnS-Mn thin films on plural glass substrates having a large area.
In the case that the thin films for EL display are formed on the glass substrates having a large area, it is necessary to improve crystallinity of the thin films by CVD method under reduced pressure in order to produce a uniform and excellent luminescent layer over a large area. For an improvement of productivity of the thin films, it is necessary to increase the number of substrates on which the thin films grow simultaneously, to narrow a distance between the substrates within the limits which maintain uniformity of the thin films and to accelerate a growth rate of the thin films.
Studies and investigations on the growth of ZnS thin film by means of CVD technique have been pursued from the 1950s. For the purpose of applying the ZnS thin film to various uses such as luminescent elements, solar cells, photoconductive wave paths, optoelectronic integrated circuits and the like, the ZnS thin film has been prepared by epitaxial growth of ZnS single crystal at a high temperature (700.degree.-800.degree. C.) under atmospheric pressure. Although an application of the ZnS thin film to the substrate having a large area was not investigated at the commencement, non-uniformity of the film thickness came into question as the size of the wafer became larger in the field wherein thin films of Si or GaAs and the like prepared by epitaxial growth were utilized. It has been known that a film quality can be improved effectively by the CVD method under reduced pressure. Studies on film growth of ZnS by CVD under reduced pressure have been carried out from the 1970s. For example, it is described in Japanese Patent Opening No. 7715/1972 that ZnS film can be grown effectively by CVD at about 450.degree. C. under a gas pressure of 50-300 Torr. It is known that polycrystalline ZnS film can be grown by CVD under a reduced pressure of 0.1-10 Torr (Japanese Patent Publication No. 47717/1985). According to this film growth method by CVD under a reduced pressure, an optimal gas pressure changes depending on various factors such as the kind and concentration of material gases to be used, the size and form of a substrate, the size and structure of an apparatus and the like, and a growth rate of ZnS film decreases as the gas pressure becomes lower. For these reasons, appropriate conditions for preparing ZnS films having desired properties have not been investigated.